The present invention relates to a process for producing miniature piezoelectric devices using laser machining and devices obtained by this process.
A piezoelectric device can be in various forms. One of the simplest forms consists of a thin plate or wafer metallized on both faces. Other forms or shapes of varying complexity are possible, such as a sequence of plates connected by bridges, tuning forks, etc. The essential property used in such devices is their resonance. However, coupled resonators can lead to filtering properties of the band pass type.
If a filter is formed from several coupled piezoelectric resonators and if said resonators are machined in the same piezoelectric substrate a so-called monolithic filter is obtained. By analogy, a polylithic filter is obtained by the cascade association of several monolithic cells.
The term piezoelectric devices is understood to means all said class of means covering piezoelectric resonators, piezoelectric filters, of either the monolithic or polylithic type, as well as any other combination of such elements having a given transfer function.
Such devices have a very wide range of applications:
particularly mobile, fixed reception, medium frequency filtering, PA1 filtering for radio reception antenna (for increasing the signal-to-noise ratio), PA1 fixed or mobile instrumentation, construction of oscillators with a large frequency sweep (VCXO), PA1 electronic watches and clocks. PA1 the article entitled "Low frequency resonators of lithium tantalate" by ONOE SHINADA (Kinsekisha Ltd.), 1973, p. 42; PA1 the article entitled "Miniature LiTaO.sub.3 X-cut strip resonator", 1983, by OKAZAKI, WATANABE (NDK), p. 337; PA1 and finally the article entitled "Miniaturized LiTaO.sub.3 strip resonator", 1983, by FUJIWARA (Fujitsu), p. 343. PA1 (1) the cutting of the pattern is very precise; PA1 (2) the loss of material is very small, because the laser beam is focused on to a spot whose diameter does not exceed 15 .mu.m (it is difficult to drop below this dimension because the thickness of the crystal to be cut is approximately 200 .mu.m); PA1 (3) the acoustic quality of the devices obtained is not inferior to that obtained with devices produced by sawing; PA1 (4) the piezoelectric quality is also not inferior, the proportion of degraded material in the vicinity of the cut being the same as that in the case of sawing, which is confirmed by limited ageing over a period of several months and an electromechanical coupling coefficient value which is very close to that of the bars produced by sawing; PA1 (5) the dimensional reproducibility is excellent so that it is possible to come very close to the final characteristics of the devices whilst reducing the regulating and setting phase; PA1 (6) the dimensions and shapes can be adjusted with the very laser used for cutting, if it is wished to optimize the various operating parameters or reduce the parasitic modes. PA1 maximum relative band width obtained without linked compensated elements: approx. 2% for an undulation or ripple of 0.2 dB; PA1 insertion loss for four resonators: &lt;0.05 dB; PA1 rejection of interference in an attenuated band &gt;60 dB for a monolithic cell and &gt;90 dB for the combination of two or more cells; PA1 overvoltages obtained on resonators and filters: PA1 the two faces are then metallized through a mask, with the exception of the coupling bridges 21, 22, 23 (FIG. 1b); PA1 sockets are then fitted on each face in the centre of the end bars A and B (16.sub.A, 18.sub.A, 16.sub.B, 18.sub.B); PA1 the intermediate resonators are electrically short-circuited; PA1 part 14 is cut with a laser to free the pattern; PA1 the pattern is fitted in its box; PA1 it is adjusted or set using the same laser by machining the ends of the bars and the width of the couplers 21, 22, 23; PA1 the box is then sealed under vacuum; PA1 the device is adapted to obtain the final characteristics of the filter. PA1 6.32.times.1.7 mm at 455 kHz PA1 3.16.times.1.1 mm at 1 MHz PA1 1.5.times.0.6 mm at 2 MHz
The processes for producing such devices, when they are of the miniature type, use two different techniques, as a function of whether the piezoelectric material used is quartz or one of the new materials constituted by lithium niobate and tantalate.
In the case of quartz, the procedure used is photolithography. A photosensitive resin is deposited on a quartz substrate and is irradiated for forming lines corresponding to the pattern to be cut, which is followed by chemical etching of the quartz, e.g. using a boiling hydrofluoric acid solution.
In the case of lithium niobate or lithium tantalate, this procedure is unsuitable, because the chemical etching speed of these materials by the acid is much too low. Thus, the etching speed of quartz by boiling hydrofluoric acid is approximately 74 .mu.m/hour, whereas that of lithium tantalate by a hydrofluoric and nitric acid solution is only 1.6 .mu.m/hour.
Thus, in the case of lithium tantalate, photolithography is considered to be unsuitable because, in view of the thicknesses of the wafers to be machined (200 .mu.m) there would be a significant underetching phenomenon, which would lead to a poor definition of the edges of the patterns. Moreover, the photosensitive resin would be destroyed.
For this reason, preference is given to the use of another procedure, i.e. sawing. For this purpose, use is made of a diamond saw having multiple blades or wires.
With regards to the lithium tantalate piezoelectric devices, reference can be made to a number of articles which appeared in the journal "Symposium on Frequency Control", namely:
The characteristics of the resonators and filters produced from lithium niobate and tantalate are complementary of those of those of their quartz homologs. However, for resonators, the frequency sweep is greater and for filters the relative pass band is wider. Their range of use varies between a few dozen kHz to approximately 30 MHz, and this can be broken down into a number of ranges as a function of the crystal section chosen.
However, despite the interest provided by lithium niobate and tantalate, it must be omitted that they are not very suitable for obtaining complex shapes, due to the sawing procedure used. Thus, it is necessary to make do with very simple shapes, such as rectangular bars or tuning forks, which does not make it possible to easily optimize certain parameters, such as the overvoltage. The sawing problems also prevent the production of monolithic filters in the sense defined hereinbefore. Thus, it is not possible when using a saw to remove material in closed areas with often millimetric and submillimetric dimensions. In order to obtain filters with several resonators in cascade, it is necessary to separately produce resonators in different piezoelectric substrates and then assemble these different devices by electrical connections. Thus, there are no lithium niobate or lithium tantalate multi-resonator monolithic filters.
Moreover, sawing implies that the part to be machined has relatively large dimensions, due to the fragility of the crystals, so that it is difficult to reach high operating frequencies (beyond 1 MHz) for crystal sections using elongation modes.
A further difficulty is presented by the regulation or setting of the devices obtained. No matter whether the production method involves photolithography or sawing, its reproducibility is often of a random nature, and the device obtained only rarely has the expected characteristics. It is then necessary to carry out an adjustment (e.g. of the resonant frequency). This involves filing by using several different methods, some parts of the pattern, particularly the ends. Obviously this operation is far from easy and considerably extends the production time.
Finally, the prior art sawing procedure is not appropriate for programming production installations, which is indispensable if the production process it to be given a true industrial character.